In order to achieve a maximum turbine output, it is advantageous to work at the highest possible gas temperatures. In modern gas turbines, the temperatures are so high that many components have to be cooled, since otherwise the temperature of the components which is permissible for maximum durability would be exceeded. A suitable design and/or cooling of critical components is therefore of crucial importance in modern gas turbines. The cooling problem of platforms occurs to an increased extent in annular combustion chambers, since the latter produce a very uniform temperature profile at the entry to the turbine. This means that the platform of the blade has to bear almost the average hot-gas temperature. To achieve the lowest possible NOx emissions, virtually the entire proportion of the combustion air is delivered through the burners themselves in modern combustion chambers; the proportion of the cooling air for the film cooling of the combustion chamber is therefore reduced. This likewise leads to a more uniform temperature profile at the turbine entry and thus to increased thermal loading.
Critical components in turbines are, inter alia, heat shields, combustion-chamber segments and combustion-chamber plates, moving and guide blades, inner and outer shroud bands of the moving and guide blades, and also moving- and guide-blade platforms.
In particular at the sides of segments (platforms) arranged next to one another, experience shows that increased thermal loading often occurs. If, for instance, the segments of a platform are coated with a heat-insulating coating, peeling of the coating is often found. This results in a weak point, at which oxides rapidly form, and these oxides in turn encourage the peeling of the coating. Large uncoated metal surfaces can thus be subjected to the hot-gas stream in a short time.